Television and like camera tubes



Jan. 10, 1961 w. E. TURK 2,967,962

TELEVISION AND LIKE CAMERA TUBES Filed Oct. 25, 1958 PHOTO CATHODE COATING MATERIAL FIG Z PHOTO CATHODE\ 5 SIGNA PLATE LAYER n PLOROUS LAYER 5- COMPACT LAYER INVENTOR 1 mm W 72M Zia/a m? KIWI? L8 ATTORNEYS TELEVISION AND LIKE CAMERA TUBES Walter Ernest Turk, Chelmsford, Essex, England, as-

signor to English Electric Valve Company Limited, London, England, a British company Filed Oct. 23, 1958, Ser. No. 769,252

Claims priority, application Great Britain Jan. 6, 1958 Claims. (Cl. 313-65) This invention relates to television and like camera tubes and more specifically to such tubes of the kind in which there is a field-terminating mesh grid fairly close to the picture receiving photo-cathode of the tube. A very well known camera tube of this kind is the socalled Vidicon tube. In this type of tube the photocathode is formed on the inside of the end wall of the envelope and, fairly close to and parallel to the said photo-cathode, is a mesh or grid which lies transversely across the otherwise open end of the tubular metal focusing electrode system.

In Vidicon and other camera tubes of the kind referred to, the photo-cathode on the inside of the end wall of the tube is usually formed in two stages as follows: Firstand after a transparent signal plate layer (usually of tin oxide) has been deposited on the glassa crucible containing antimony trisulphide with from 2% to 6% excess antimony is introduced through a branch pipe on the envelope so as to lie inside the tubular metal focusing electrode system and the crucible is electrically heated to evaporate its contents and deposit the evaporated material through the mesh on to the end wall, i.e. onto the signal plate layer. This evaporation step is performed in the presence of an inert gas, usually argon, and results in the deposition of a porous or somewhat spongy layer on the end wall. It also results in a similar deposit on the mesh itself and since this deposit is (in the absence of light irradiation) insulating it must be removed. Accordingly the crucible is now removed, the envelope is pumped out to a hard vacuum and eddy current heating is applied to the mesh to evaporate the deposit thereon. This second evaporation step forms a second deposit on the end wall, this time of a compact substantially non-porous nature. However, it leaves the mesh, which is normally of copper, with a bright, light reflecting surface and experiment has now shown that, due to the comparative proximity of the mesh to the cathode, this often has a deleterious effect upon subsequent operation of the tube. It is one object of the present invention to avoid this defect. It is a further object to provide camera tubes with improved photo-cathodes.

According to this invention the mesh or grid of a camera tube of the kind referred to is initially coated,

either directly or upon an intermediate coating layer,

with a metal of low light reflecting quality. Preferably the metal is antimony. By initially coated is meant that the coating is applied to the mesh before the evaporation steps by which the photo-cathode is deposited on the end wall of the tube.

According to this invention from another aspect a process of making a photo-cathode in a camera tube of the kind referred to, comprises the steps of initially coating the mesh with a metal of low light reflecting quality (preferably antimony) either directly or upon an intermediate metal layer, evaporating antimony trisulphide through said mesh in the presence of an inert gas to form a porous cathode layer and then evaporating 2,957,962 Patented Jan. 10, 1961 has shown that, in fact, the result of providing this in-' itial coating is to leave the mesh, after the said evaporation step, dark and of poor light reflecting qualities despite that one would expect the said step of evaporation to drive oif the initial coating and leave the mesh bright. Whatever the explanation may be, in fact the said step does not leave a bright mesh.

Where the initial coating is applied on an intermediate coating, instead of directly on the mesh, the intermediate coating is preferably chromium.

As already stated, known present day practice in manufacturing tubes of the kind referred to, involves the evaporation, in two stages, of antimony trisulphide with 26% excess antimony. It has been found possible and advantageous, in carrying out the present invention, to provide the mesh With an initial layer of antimony so chosen in amount, as to permit the antimony trisulphide introduced in the crucible and evaporated therefrom to be stoichiometrically pure, the necessary excess antimony required to make the deposit photo-electric being obtained by evaporating antimony from the initial deposit on the mesh. This expedient has been found to give better control of the constituents of the deposited photo-electric layer and to result in a more satisfactory photo-cathode.

The invention has been found to give another important advantage and experiment has shown, that the invention results in photo-cathodes of greater uniformity than cathodes produced by methods as at present practised. The inclusion of a metal oxide in the coating on the mesh results also in increased sensitivity and lower dark current. For this reason it is preferred to produce the coating by evaporating or sputtering on the coating materialpreferably antimonyin an atmosphere of oxygen. This produces a film of oxide on the mesh, with resultant improved sensitivity and lower dark currents.

The invention is illustrated in the accompanying drawings in which Figure 1 shows schematically, and only so far as is necessary to an understanding of the invention, one way of carrying the said invention into effect; Figure 2 is a fragmentary cross section of a signal plate, drawn on an enlarged scale; and Figure 3 is a fragmentary cross section of a field terminating mesh, also drawn on an enlarged scale.

Referring to the drawing, 1 is part of the glass envelope of a Vidicon television camera tube. This envelope has a fused in glass branch pipe 2. The end of the envelope is 7 closed by the usual flat end glass plate 3 on the inside of which the photo-cathode, generally designated 5, is to be deposited on top of a signal plate deposit 11 constituted by tin oxide. Contact is made to the signal plate in known manner by means of a contact ring 4 of Kovar or other suitable metal. Inside the envelope is the usual metal tubular focusing electrode system 6, the otherwise open end of which is closed by a field terminating mesh 7. The tubular metal electrode 6 has a side opening 8 opposite the branch pipe 2. In accordance with this invention the mesh 7 is coated at 12, as by evaporating antimony thereon, before being put in place.

Through the branch pipe 2 and the hole 8 there is introduced a crucible 9 containing a measured quantity of antimony trisulphide. This may be, as is at present customary, antimony trisulphide with from 2 to 6% excess antimony, but, by suitably choosing the amount of antimony initially deposited on the mesh stoichiometrically pure antimony trisulphide may be, and preferably is, used. The crucible is heated in the presence of an argon filling by electrical heating applied through leads in the crucible support 10 so as to evaporate its contents and produce a porous deposited layer through the mesh on the signal plate 11 on the inside of the glass end plate 3. It also produces a deposited layer on the mesh itself. The crucible is then withdrawn, the envelope is pumped out to a hard vacuum, and the branch pipe 2 is sealed off to leave a mere side pipe on the envelope. A high frequency eddy current heating coil (not shown) is now placed round the tube to encircle the mesh and heat is applied to raise the mesh above the evaporation temperature of antimony. This evaporates off the antimony trisulphide p'roviously deposited on the mesh and provides a second, compact, photo-cathode layer 5" on the plate 3. Although this evaporation step is conducted at such temperature that one would expect the antimony also to be evaporated from the mesh, the fact is that the mesh remains practically dark after the said strip. It is not known whether antimony in fact remains on the mesh or whether it becomes compounded therewith in some way and the full explanation of some of the benefits obtained is obscure. Whatever the explanation may be, however, experiment shows that the mesh remains dark and almost completely non-reflecting of light, while the photo-cathodes of high sensitivity, good uniformity and low dark current are obtainable.

I claim:

l. A method of manufacturing a camera tube having a picture receiving photo-cathode with field terminating mesh adjacent thereto, said method including the steps of initially coating the mesh with a metal of low light reflecting quality, evaporating antimony trisulphide through said mesh in the presence of an inert gas to form a porous cathode layer, and then evaporating antimony trisulphide from said mesh to form a second compacted cathode layer.

2. A method as claimed in claim 1 wherein the metal of the coating is antimony.

3. A method as claimed in claim 1 wherein the coating is deposited directly on the mesh.

4. A method as claimed in claim 1 wherein the coating is deposited on an intermediate layer on the mesh.

5. A method as claimed in claim 1 wherein the intermediate layer is of chromium.

6. A method as claimed in claim 1 wherein the coating contains oxygen.

7. A method as claimed in claim 1 wherein the initial coating is effected by sputtering on the metal in an atmosphere of oxygen.

8. A method as claimed in claim 1 wherein the initial coating is effected by evaporating on the metal in an atmosphere of oxygen.

9. The method of manufacturing a camera tube having a picture-receiving photo-cathode with a field terminating mesh adjacent thereto which includes fusing a branch tube in one side of the camera tube envelope, introducing a tubular electrode having a side opening therein into the enve.ope with the side opening of the electrode registering with the branch tube coating the mesh with a layer of metal of low light reflecting quality, introducing a measured quantity of antimony trisulphide through the branch tube and the registered side opening in said electrode, heating the antmony trisulphide in the presence of an argon filling within the envelope to produce a porous deposited layer on the terminating mesh, pumping out the envelope to a hard vacuum, sealing off the branch tube form the envelope, subjecting the mesh to a temperature above evaporation temperature of antimony for evaporating oil the antimony trisulphide previously deposited on the mesh and building up a photo-cathode layer on the picture-receiving photo-cathode.

10. A camera tube including a picture receiving photocathode with a field terminating mesh adjacent thereto wherein said mesh has an initial coating of metal of low light reflecting quality.

Great Britain Nov. 5, 1952 France Mar. 11, 1957 

